1. Field of the Invention
This invention relates generally to rhodium catalysts and to the preparation thereof, and more particularly concerns rhodium catalysts that are suitable for use in the purification of crude terephthalic acid and to the preparation of such catalysts.
2. Discussion of the Prior Art
Polymer grade or "purified" terephthalic acid is the starting material for polyethylene terephthalate, which is the principal polymer used for making polyester fibers, polyester films, and resins for bottles and the like containers. Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in Meyer, U.S. Pat. No. 3,584,039 or Stech et al., U.S. Pat. No. 4,405,809. In the purification process, the impure terephthalic acid is dissolved in water at an elevated temperature and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst containing a noble metal, typically palladium, on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915. This hydrogenation step converts the various color bodies present in the relatively impure terephthalic acid to colorless products.
Puskas et al., U.S. Pat. Nos. 4,394,299 and 4,467,110 disclose the use of a combination noble metal catalyst, for example, a palladium/rhodium catalyst on a porous carbonaceous surface, for purification of aqueous terephthalic acid solutions. The latter two patents also show the use of a rhodium-on-carbon catalyst under reducing conditions and review various heretofore known methods of preparing a Group VIII metal catalyst having activity and selectivity suitable for the purification of terephthalic acid by hydrogenating its principal impurity, 4-carboxybenzaldehyde, to p-toluic acid.
However, p-toluic acid is also an impurity that must be removed from the hydrogenated aqueous terephthalic solution. While such removal can be achieved to a large extent owing to the greater solubility of p-toluic acid, as compared to terephthalic acid in water, nevertheless substantial amounts of p-toluic acid are trapped within purified terephthalic acid crystals as the hydrogenated terephthalic acid solution is crystallized to recover purified terephthalic acid.
More recently it has been discovered in our laboratories and disclosed in copending U.S. patent application Ser. Nos. 785,321 and 785,322 concurrently filed with U.S. patent application Ser. No. 785,055, the parent of the present application, that the amount of p-toluic acid produced during purification of aqueous crude terephthalic acid solutions can be minimized by the use of a catalyst system in which different layers of an aforesaid palladium-on-carbon catalyst and of a rhodium-on-carbon catalyst are employed where the rhodium metal is supported on particulate active carbon. Such a catalyst system does not promote hydrogenation of 4-carboxybenzaldehyde to p-toluic acid but, instead, decarboxylates 4-carboxybenzaldehyde to benzoic acid, a by-product that is more soluble in water than p-toluic acid and thus is more readily separable from terephthalic acid upon crystallization of the latter.
Furthermore, it has been disclosed in Schroeder and James, U.S. patent application Ser. No. 905,766 filed Sept. 10, 1986 entitled "Method of Purification of Terephthalic Acid and Means Therefor" that the concurrent use of the aforesaid rhodium-on-carbon catalyst in a first layer and an aforesaid conventional palladium-on-carbon catalyst in a second layer in the purification reactor and the passage of a solution of crude terephthalic acid in sequence through the first layer and then through the second layer and in which the crude terephthalic acid is purified effects a decrease in the concentration of color bodies and fluorescent impurities in the resulting purified terephthalic acid, relative to the use of the conventional palladium-on-carbon catalyst alone.
However, commercially available rhodium-on-carbon catalysts tend to decompose considerable amounts of terephthalic acid to benzoic acid, thereby reducing the yield of purified terephthalic acid. Thus, it would be desirable to employ a rhodium-on-carbon catalyst that is selective toward decarbonylation of 4-carboxybenzaldehyde without the attendant conversion of substantial amounts of terephthalic acid to benzoic acid.
It has now been discovered that a rhodium-on-carbon catalyst having an enhanced selectivity for decarbonylation of 4-carboxybenzaldehyde to benzoic acid under reducing conditions and enhanced activity for the removal of color bodies and fluorescent impurities in the purification of crude terephthalic acid can be produced.